Field of the Disclosure
The present disclosure relates generally to leaf springs for vehicle suspensions and to a method of manufacturing the same. The disclosure presents several example embodiments that may be utilized for particular purposes.
Description of Related Art
In the past, quenched and tempered steel leaf springs for trucks and other heavy duty vehicles have been specified with a finished hardness, such as, between 375 BHN and 461 BHN (Brinell hardness number). More recently, leaf springs in Europe and Japan have been manufactured with a higher specified hardness, such as, 461 BHN to 514 BHN. These higher hardness leaf springs show an improvement in fatigue life.
The demand for higher hardness leaf springs is reinforced at least in part by the desire to reduce vehicle weight and in particular, unsprung suspension system weight. The higher hardness leaf springs allow for the use of fewer and/or thinner and lighter leaf springs relative to more traditional, lower hardness leaf springs referred to above. As a result, fuel economy as well as control, performance and efficiency of the suspension system improve. In addition, new laws require trucks and other heavy duty vehicles to be capable of stopping in shorter distances, imposing greater demands on a suspension system.
While higher hardness leaf springs show an improvement in fatigue life, there has been, however, a persistent, low, but nevertheless increased incidence in early failures, particularly of the main leaf of a suspension system at the eyes when compared with springs that are quenched and tempered to traditional hardness ranges. Similar failures have also occurred at or around the center or other bolt hole, if present, in the seat of the leaf spring. These failures are the result of hydrogen environment assisted cracking (HEAC), also known as hydrogen assisted cracking.
Hydrogen assisted cracking can occur in high strength steels when three conditions are met: 1) a condition of static assembly stress such as may occur as the result of clamping forces at a seat or hoop stress from insertion of a bushing into an eye; 2) the existence of a galvanic couple sufficient to charge the steel with hydrogen; and 3) the steel involved is of sufficient strength to trigger the mechanism of failure.
Hydrogen assisted cracking has a peculiarity in that as the strength of the steel increases, the threshold stress required to trigger hydrogen assisted cracking goes down, thus creating a disadvantageous, inverse relationship.
In light of the foregoing, the current state of leaf springs and in particular high hardness leaf springs, given the strong correlation in steels between hardness and strength, leaves something to be desired.